The use of activated carbon for the purification of drinking water, the tertiary treatment of sewage water, and, generally, the industrial use of activated carbon to remove impurities from process streams is well known. Industrial wastes, particularly, those containing organic materials that are not easily biodegraded are usefully removed from process streams by adsorption on activated carbon. However, to be economically feasible for use on a large scale, the activated carbon after it has become saturated with organic material must be capable of regeneration in order to allow its reuse. By the methods of the prior art the activated carbon can be regenerated either thermally or by solvent extraction or discarded, for instance, in a landfill. Activated carbons contain a high surface area and are available in various forms including powder and granular activated carbons, carbon felts, and two dimensional activated carbon cloth.
The prior art method of thermal regeneration of activated carbon which has become saturated with organic pollutants requires that the material be shipped to a specialized facility where the material is heated to about 900.degree. C. in an atmosphere of reduced oxygen. The organic compound pollutants are destroyed by incineration and the carbon is reactivated thereby. However, about 10 to about 15 percent by weight of the carbon is lost by oxidization during the thermal regeneration process. Steam thermal regeneration is also practiced in a specialized facility utilizing lower temperatures and, hence, less carbon is lost by oxidation during processing. During the steam thermal regeneration method, the organic pollutant is separated from the carbon but is not destroyed. Solvent extraction methods for regeneration of activated carbon also remove the organic contaminant from the carbon but in this process about 10 to about 15 percent of the pores of the active carbon are blocked by the extraction solvent which remains behind subsequent to the solvent extraction process.
Accordingly, the primary object of this invention is to provide a new and improved method of regenerating activated carbon and polymeric adsorbents subsequent to saturation to allow reuse without any substantial loss of activity. Still another object of the invention is to provide a method of enhancing the adsorption capacity of activated carbon utilizing an apparatus which is also effective in regenerating the activated carbon.
A method is disclosed in U.S. Pat. No. 4,260,484 to Connolly for renewing the adsorptive capacity of a bed of active carbon. In this process, a bed of activated carbon saturated with an hydroxyaromatic compound is made the positive electrode of an electrolytic cell. A positive potential is applied to the carbon in order to transform the adsorbed organic compound into a species which has either a high affinity for the carbon or a low affinity for the conducting solution of the electrolytic cell. Thereafter, the applied potential is withdrawn and an effluent containing hydroxyaromatic compounds is introduced to the bed of activated carbon. Additional compounds can now be removed from the effluent. Connolly cites various patents directed to the electrochemical treatment of effluents or activated carbon, namely U.S. Pat. No. 3,730,864 to TarJanyi et al., U.S. Pat. No. 3,730,885 to Makrides et al. and U.S. Pat. No. 4,131,526 to Moeglich. Each of these cited patents relate to the electrochemical treatment of an organic material either as an aqueous solution or adsorbed on a mass of active carbon.
In U.S. Pat. No. 4,217,191 to Doniat et al., a process is disclosed for regenerating activated carbon particles contaminated by oxidizable impurities by suspension of such particles of activated carbon in an aqueous solution of an electrolyte which is loaded into an electrolytic cell in which a voltage is applied to the electrodes of said cell so as to liberate atomic oxygen at the surface of an anode. As the suspension of particles of activated carbon is passed through the anode compartment of the cell, the particles contact the anode electrode and the atomic oxygen generated thereon. The oxidizable impurities are destroyed and the activated carbon is regenerated.
In U.S. Pat. No. 5,414,204 to Hosono et al., activated carbon having an organochlorine compound adsorbed thereon is reactivated by treatment with ionizing radiation which results in decomposition of the organochlorine compound previously adsorbed on the activated carbon. The carbon can be further used after subsequent treatment to activate the carbon.
In U.S. Pat. No. 5,232,484 to Pignatello, a method is disclosed for degrading an organic pesticide present in an acidic, aqueous solution in the presence of light by reaction with an hydroxyl radical produced by the reaction of a ferric ion and a peroxide, preferably, hydrogen peroxide.
In U.S. Pat. No. 4,834,852 to Wabner, a process is disclosed for the oxidation of organic substances in an aqueous medium by exposure to an hydroxyl radical (Fenton's reagent) formed by the reaction of a mixture of hydrogen peroxide and iron (II) salts. In the process of Wabner, hydrogen peroxide is continuously added to an electrolytic cell which is supplied with direct current to achieve a current density of about 0.4 to about 50 milliamps per square centimeter. The process of the invention converts difficultly degradable or toxic substances in an aqueous medium into biologically degradable or degraded substances. The cathode material of the electrolytic cell of Wabner can be titanium, nickel, graphite, synthetic carbon or valve materials such as titanium, zirconium, and tantalum which have been coated wholly or partially with a platinum metal. The anodes can be graphitic or dimensionally stable valve metal anodes. The pH of the electrolyte of the cell of Wabner can be 0 to about 14, although, generally, the oxidation reaction takes place more quickly at pH values of less than 8. Instead of oxidation with Fenton's reagent, Wabner also suggests the use of a direct current in the electrolytic cell to activate the hydrogen peroxide so as to cause oxidation of the degradable or toxic substances. Fenton's reagent is also disclosed in U.S. Pat. No. 5,538,636 to Gnann et al. as useful in a process for chemically oxidizing waste water.
In The Journal of Applied Electrochemistry 23 (1993) Hsiao et al. disclose the use of Fenton's reagent, produced by the reaction of ferric salts, with electrolytically generated hydrogen peroxide to produce hydroxyl radicals which are used to oxidize chlorobenzene and phenol. Reaction with electrogenerated Fenton's reagent in an electrolytic cell reduces the concentrations of chlorobenzene and phenol in the electrolyte. The electrolytic cell used for the reaction contains a reticulated, vitreous carbon cathode and a platinum anode. Electrolysis is conducted at a current density of 0.7 milliamps per square centimeter. The three hydroxylated reaction products obtained are identified as catechol, resorcinol, and hydroquinone with catechol being the dominant product. Further work illustrating the versatility of the hydroxyl radical in oxidation reactions with organic compounds is described in Accounts of Chemical Research, 8, 125, 1975, "Fenton's Reagent Revisited", Walling, C.
It is known that other transition metals besides iron can be used to produce hydroxyl radicals from hydrogen peroxide in aqueous solution. Examples of these agents include selected aqueous ions of the following metals: titanium, chromium, copper, and tin. Other transition metals which exhibit variable ionic oxidation states in aqueous solution may also be useable.
Other methods of producing hydroxyl radicals are described in Chemical Engineering, September 1994, pages 16-20 under the terms Advanced Oxidation Processes, incorporated herein by reference. In addition to using a Fenton's reagent system to generate hydroxyl radicals, the reaction of hydrogen peroxide with ultraviolet light or ozone is known to produce hydroxyl radicals. In U.S. Pat. No. 4,792,407 a combination of ultraviolet radiation, ozone, and hydrogen peroxide is used to oxidize organic compounds in an aqueous system.